bioRxiv preprint doi: https://doi.org/10.1101/2021.08.04.455130; this version posted August 5, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 1 ALLELOPATHY AS AN EVOLUTIONARILY STABLE STRATEGY 2 3 Rachel M. McCoy1,2, Joshua R. Widhalm1,2, and Gordon G. McNickle1,3,* 4 5 1Purdue Center for Plant Biology, Purdue University, West Lafayette, IN 47907, USA. 6 7 2Department of Horticulture and Landscape Architecture, Purdue University, 625 Agriculture Mall 8 Drive, West Lafayette, IN 47907, USA. 9 10 3Department of Botany and Plant Pathology, Purdue University, 915 West State Street, West 11 Lafayette, IN 47907, USA. 12 13 *Correspondence: G.G. McNickle, Center for Plant Biology and Department of Botany and Plant 14 Pathology, Purdue University, 915 West State Street, West Lafayette, IN 47907, USA. 15 ([email protected]) fax + 1 765 494 0363, tel. +1 765 494 4645 16 17 Running title: Allelopathy as an evolutionarily stable strategy 18 1 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.04.455130; this version posted August 5, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 19 ABSTRACT 20 In plants, most competition is resource competition, where one plant simply pre-empts the 21 resources away from its neighbours. Interference competition, as the name implies, is a form of 22 direct interference to prevent resource access. Interference competition is common among 23 animals who can physically fight, but in plants, one of the main mechanisms of interference 24 competition is Allelopathy. allelopathic plants release of cytotoxic chemicals into the 25 environment which can increase their ability to compete with surrounding organisms for limited 26 resources. The circumstances and conditions favoring the development and maintenance of 27 allelochemicals, however, is not well understood. Particularly, it seems strange that, despite the 28 obvious benefits of allelopathy, it seems to have only rarely evolved. To gain insight into the 29 cost and benefit of allelopathy, we have developed a 2 × 2 matrix game to model the interaction 30 between plants that produce allelochemicals and plants that do not. Production of an 31 allelochemical introduces novel cost associated with synthesis and detoxifying a toxic chemical 32 but may also convey a competitive advantage. A plant that does not produce an allelochemical 33 will suffer the cost of encountering one. Our model predicts three cases in which the 34 evolutionarily stable strategies are different. In the first, the non-allelopathic plant is a stronger 35 competitor, and not producing allelochemicals is the evolutionarily stable strategy. In the 36 second, the allelopathic plant is the better competitor and production of allelochemicals is the 37 more beneficial strategy. In the last case, neither is the evolutionarily stable strategy. Instead, 38 there are alternating stable states, depending on whether the allelopathic or non-allelopathic 39 plant arrived first. The generated model reveals circumstances leading to the evolution of 40 allelochemicals and sheds light on utilizing allelochemicals as part of weed management 41 strategies. In particular, the wide region of alternative stable states in most parameterizations, 42 combined with the fact that the absence of allelopathy is likely the ancestral state, provides an 43 elegant answer to the question of why allelopathy rarely evolves despite its obvious benefits. 44 Allelopathic plants can indeed outcompete non-allelopathic plants, but this benefit is simply not 2 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.04.455130; this version posted August 5, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 45 great enough to allow them to go to fixation and spread through the population. Thus, most 46 populations would remain purely non-allelopathic. 47 48 Keywords: allelopathy; game theory; evolutionarily stable strategy; modeling 49 50 INTRODUCTION 51 Competition is ubiquitous in the natural world, as there are finite resources available in a 52 given time and space1–3. Thus, competition generally reduces plant fitness when resources, such 53 as light, space, water and nutrients are limiting4,5. This type of competition for finite resources is 54 broadly named resource competition and occurs when organisms compete by simply reducing 55 the availability of resources to other organisms6. Alternatively, interference competition occurs 56 when one organism interferes with, and therefore reduces, the ability of the other to obtain a 57 shared resource while not necessarily drawing down resource concentrations6. Animals routinely 58 face interference competition as they can physically fight over resources7. Sessile plants primarily 59 compete via resource competition. However, one of the major mechanisms of interference 60 competition in plants is mediated chemically through allelopathy8. For example, one of the best 61 documented examples is allelopathy by walnut trees (Juglans spp.), mediated by the 62 allelochemical juglone, which is toxic to a variety of crop and horticultural species, including corn 63 and soybean12 and tomato and cucumber13. 64 65 Allelopathy is the production of chemicals, called allelochemicals, that are released into 66 the environment and negatively affect the growth and development of competing individuals9. 67 Although the term was first used in 1937, the effect has been recognized for thousands of years9. 68 Unfortunately, there have been difficulties in studying the competitive effects of allelopathy 69 because of methodological difficulties. For example, for many years experiments used soil 70 additives such as activated charcoal that were thought to prevent the activity of allelochemicals 3 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.04.455130; this version posted August 5, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 71 with the goal of comparing how plants grew either with or without the presence of this form of 72 interference competition. Unfortunately, it was later learned that activated charcoal also stimulates 73 nutrient availability, and thus, many years of research showing the negative effects of 74 allelochemicals were probably just detecting the positive effects of fertilization (e.g.10,11). 75 76 Despite limitations in the ability to experimentally study allelopathy, it has been implicated 77 in the success of some invasive plants, highlighting the advantage of interference competition as 78 a strategy14. Invasion by non-native species is ranked the second strongest risk to natural 79 diversity15. For example, Paterson’s curse (Echium plantagineum L.) is an invasive weed in 80 Australia, affecting up to 30 Mha, whose invasion success is partially attributed to production of 81 the allelochemical shikonin and its derivatives16. Indeed, one commonly invoked mechanism for 82 invasion by non-native species is the novel weapons hypothesis, which suggests invasive species 83 are successful through use of competitive strategies for which native species have not co-evolved 84 counter strategies17,18. This mechanism has been linked to the invasion success of allelopathic 85 Policeman’s helmet (Impatiens glandulifera)19, which releases a compound structurally similar to 86 shikonin called 2-methoxy-1,4-naphthoquinone (2-MNQ) that elicits negative effects on herb 87 germination and mycelium growth and is otherwise absent in soils without I. glandulifera, thus 88 suggesting 2-MNQ may function as a “novel weapon”19–21. From these studies, it may be possible 89 that allelochemicals may have significant potential for genetically modified cropping systems to 90 enhance the competitive ability of crop species over weeds. 91 92 Despite the potential advantages of allelochemicals as an evolved tool for interference 93 competition, they seem to have only rarely evolved. Here, we report an evolutionary game 94 theoretic model to probe the benefits and circumstances that might favor the evolution of 95 allelochemicals to better understand why they might not be more common in plants. Specifically, 96 we ask: 1) What circumstances favor the production of allelochemicals? 2) How does the cost of 4 bioRxiv preprint doi: https://doi.org/10.1101/2021.08.04.455130; this version posted August 5, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 97 producing an allelochemical affect fitness of the plant producing the allelochemical and plants 98 competing with that plant? 3) When will allelopathic plants be stable in a population? Beyond the 99 implications for evolutionary ecology, understanding the evolution of allelopathy has the potential 100 to